SIMBAD references

A statistical analysis of the properties of radio sources in a 7degx7deg field centred on the Coma cluster of galaxies is presented, based on new observations. Unlike optical galaxies, radio sources are not highly concentrated towards the centre of the cluster, but there appears to be a small excess of sources at the periphery. Attempts to detect clustering of radio sources using the angular covariance function have produced a null result over all angular scales. The lack of a strong concentration of radio sources near the cluster centre suggests that galaxy number density is not the primary condition which determines the likelihood of a galaxy becoming a strong radio emitter. Statistical tests show there is no significant difference between the galaxy population around radio galaxies and that around non-radio galaxies in the cluster, suggesting that neither cluster related gravitational interactions nor interactions with the intra-cluster medium enhance the probability of formation of radio galaxies. In fact, the cluster environment seems to suppress the strong radio galaxy phenomenon. Radio source counts at 408 MHz, which have a limiting flux density of 80 mJy, are not significantly different from counts of non-cluster background sources. However, a deep source count at 1380 MHz over the central region of the cluster shows an excess in number of sources when the flux density limit is 1 mJy, suggesting that the cluster environment may induce some radio emission in galaxies, but that this emission is unlikely to be strong. The number of radio sources in the central region of the cluster which might in the future be optically identified is 15±5. Sources seen near the cluster centre show a marginally significant tendency to have steeper spectral indices than those at greater distance. Sources showing extended structure and downward curvature in their high-frequency spectra are found exclusively in the inner region of the cluster. These are considered to be sources confined by the intra-cluster medium in which spectral steepening at high frequencies is due to depletion of high-energy electrons.